Surface acoustic wave resonator

ABSTRACT

A surface acoustic wave filter includes an antenna terminal, a transmitting filter and a variable branching line both of which are coupled to the antenna terminal and a receiving filter coupled to the variable branching line. The transmitting filter has a first end series-arm SAW resonator, a second end series-arm SAW resonator, a middle series-arm SAW resonator coupled between the first and second end series-arm SAW resonators and parallel-arm SAW resonators. Each of the first and second end series-arm SAW resonator has a first resonance frequency. The middle series-arm SAW resonator has a second resonance frequency that is higher than the first resonance frequency.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to surface acoustic wave branchingfilters comprising band-pass filters having ladder constructions withsurface acoustic wave resonators, in small-size mobile communicationinstruments such as portable telephones, particularly to surfaceacoustic wave branching filters usable even in case of high appliedpowers.

[0002] In recent years, developments of small-size, light mobilecommunication instruments such as portable telephones have been rapidlyadvanced. With this, reduction in size and improvement in performance ofparts to be used have been pursued. To cope with this tendency, RF(Radio Frequency) parts using surface acoustic wave (hereinafterreferred to as SAW) filters have been developed and used. In particular,SAW branching filters of FIG. 9 have been actively developed and part ofthem have been put in practical use and used because they are devicesfor largely contributing reduction in size of RF parts.

[0003] A SAW filter includes a transmission filter (hereinafter referredto as Tx filter), a reception filter (hereinafter referred to as Rxfilter), and a branching line.

[0004] Since a high power is applied to the transmission filter, thetransmission filter must be a SAW filter whose characteristics do notdeteriorate even in the application of the high power. Such high-powerSAW filters are disclosed in, e.g., Japanese Patent ApplicationLaid-open Nos. 6-29779, 10-303682, and 11-251871.

[0005] As the filters used in the aforementioned prior arts, SAW filtersare used in each of which the transverse length or the number of pairsof SAW resonators composing its transmission filter has been increasedto increase the area per unit current and thereby raise its withstandpower.

[0006] On the other hand, a SAW branching filter in a mobilecommunication terminal device such as a portable telephone has afunction in which one antenna is used for both of transmission andreception.

[0007] Further, for this branching filter required is a function inwhich there is no change in characteristics in either of:

[0008] (1) a case that the antenna is in a normal operation, i.e., acase that the input impedance of the antenna is 50 Ω; and

[0009] (2) a case that the antenna is free, i.e., a case that the inputimpedance of the antenna is infinite.

[0010] In the above cases (1) and (2), because of a state of a rapidchange in impedance, variation and break of SAW branching filtercharacteristics upon transmission in which a power is applied to the SAWbranching filter, come into question. It is known that these variationand break of the SAW branching filter characteristics are caused bydeterioration and break of a series-arm resonator of the transmissionfilter to which a high power is applied.

SUMMARY OF THE INVENTION

[0011] In view of the aforementioned problem, the present invention maysuppress the deterioration and break of the series-arm resonator of asurface acoustic wave filter by decreasing the current flowing in theseries arm and thereby lowering the applied power to the series arm.

[0012] A surface acoustic wave filter of the present invention includesan antenna terminal, a transmitting filter and a variable branching lineboth of which are coupled to the antenna terminal and a receiving filtercoupled to the variable branching line. The transmitting filter has afirst end series-arm SAW resonator, a second end series-arm SAWresonator, a middle series-arm SAW resonator coupled between the firstand second end series-arm SAW resonators and parallel-arm SAWresonators. Each of the first and second end series-arm SAW resonatorhas a first resonance frequency. The middle series-arm SAW resonator hasa second resonance frequency that is higher than the first resonancefrequency.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a circuit diagram of a Tx filter of the first embodimentof the present invention;

[0014]FIG. 2 is a circuit diagram of a Tx filter;

[0015]FIG. 3 is a circuit diagram of an Rx filter;

[0016]FIG. 4 is a circuit diagram of a series-arm SAW resonator;

[0017]FIG. 5 is a concentrated-constant equivalent circuit diagram ofthe series-arm SAW resonator;

[0018]FIG. 6 is a circuit diagram of a SAW branching filter in a mountedstate;

[0019]FIG. 7 is a concentrated-constant equivalent circuit diagram of aTx filter; and

[0020]FIG. 8 is a concentrated-constant equivalent circuit diagram of anRx filter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Before the description of embodiments of the present invention, amounted state of a SAW branching filter will be described.

[0022]FIG. 6 is a circuit diagram of a SAW branching filter in a mountedstate.

[0023] This SAW branching filter is connected with an antenna 170 at anANT terminal 100, a transmission power amplifier 180 at a Tx terminal101, for outputting a high power, and a low-noise amplifier 190 at an Rxterminal 102, for amplifying a received small signal.

[0024] Next, the first embodiment will be described.

[0025]FIG. 1 is a circuit diagram of a Tx filter 200 of the firstembodiment of the present invention.

[0026] In the terminal device for mobile communication, such as aportable telephone, illustrated in FIG. 6, in general, the outputimpedance ZPout at a PA terminal 103 as an output terminal of a poweramplifier 180 is regulated to 50 Ω. It is known that the output power ofthis power amplifier 180 becomes, at the Tx terminal 101 of the SAWbranching filter, from the relation between the input impedance ZTin atTx and the output impedance ZPout of the power amplifier 180, a powerinput from the Tx terminal 101 of the SAW branching filter to a Txfilter 200 and a power reflected from the Tx terminal 101 of the SAWbranching filter to the PA terminal 103 of the power amplifier 180.

[0027] In the terminal device for mobile communication, such as aportable telephone, the Tx filter 200 of FIG. 6 is a four-stage T-typeladder filter made up of three series arms S1 210, S2 211, and S3 212,and two parallel arms P1 220 and P2 221 of FIG. 1. The transverselengths and the number of pairs thereof are shown in Table 1. TABLE 1Transverse length, the number of pairs, equivalent LC value, andimpedance value at 836 MHz of transmission filter Transmission ParallelParallel Parallel Parallel filter Series arm 1 Series arm 2 Series arm 3arm 1 arm 2 arm 3 arm 4 Transverse 90μ100 45μ100 90μ100 125μ85 125μ85length/the number of pairs L1 (nH) 153 306 153 87.5 87.5 C1 (pF) 0.2370.118 0.237 0.441 0.441 C0 (pF) 2.96 1.48 2.96 3.71 3.71

[0028]FIG. 3 is a circuit diagram of an Rx filter 300. As illustrated inFIG. 3, the Rx filter 300 is a six-stage π-type filter made up of threeseries arms S1 310, S2 311, and S3 312, and four parallel arms P1 320,P2 321, P3 322, and P4 323. The transverse lengths and the number ofpairs thereof are shown in Table 2. TABLE 2 Reception filter, the numberof pairs, equivalent LC value, and impedance value at 836 MHz oftransmission filter Reception Parallel Parallel Parallel Parallel filterSeries arm 1 Series arm 2 Series arm 3 arm 1 arm 2 arm 3 arm 4Transverse 50μ100 50μ100 50μ100 70μ70 99μ99 90μ90 70μ70 length/thenumber of pairs L1 (nH) 159 159 159 194 97 97 194 C1 (pF) 0.201 0.2010.201 0.181 0.362 0.362 0.181 C0 (pF) 2.51 2.51 2.51 1.77 3.55 3.55 1.77Q 800 800 800 800 800 800 800

[0029]FIG. 7 is a concentrated-constant equivalent circuit diagram ofthe Tx filter 200.

[0030]FIG. 8 is a concentrated-constant equivalent circuit diagram ofthe Rx filter 300.

[0031] In FIG. 7, the series arms S1 210 and S3 212 compose a circuit inwhich two unit circuits in each of which a capacitance CS0 is connectedin parallel with a series circuit of a reactance LS1 and a capacitanceCS1 are connected in series; the series arm S2 211 composes a circuit inwhich two unit circuits in each of which the capacitance CS1 isconnected in parallel with a series circuit of a reactance LS2 and acapacitance CS2 are connected in series; and the parallel arms P1 and P2compose a circuit in which two unit circuits in each of which thecapacitance CS0 is connected in parallel with a series circuit of areactance LP1 and a capacitance CP1 are connected in series.

[0032] In FIG. 8, the series arms S1 310, S2 311, and S3 312 compose acircuit in which a capacitance CS0 is connected in parallel with aseries circuit of a reactance LS1 and a capacitance CS1; the parallelarms P2, P3, and P4 compose a circuit in which a capacitance CP0 isconnected in parallel with a series circuit of a reactance LP1 and acapacitance CP1; and the parallel arm P1 composes a circuit in which twounit circuits in each of which the capacitance CP0 is connected inparallel with a series circuit of the reactance LP1 and the capacitanceCP1 are connected in series.

[0033] The equivalent LC value of each of the series and parallel armsis shown in Table 1. The first embodiment of the present inventionrelates to the construction of the ladder Tx filter (200) of FIG. 1,characterized in that, in the SAW branching filter of FIG. 6, upontransmission power input, the power applied to each series-arm resonatorof the Tx filter of FIG. 9 is reduced.

[0034] There is a necessity of paying attention to the power input tothe Tx filter 200 upon signal transmission. From FIG. 6, it is foundthat the input power of this Tx filter 200 is related to the inputimpedance ZTin of the Tx filter 200. That is, upon transmission, theload is in a state that the impedance ZRLin of the reception system madeup of a branching line 400 and the Rx filter 300 is connected inparallel with the ANT terminal 100.

[0035] Here is a problem that, when the power applied to each resonator210, 211, 212, 220, or 221 of FIG. 7 has risen, a current flows in thepart of the resistance between the teeth of the comb caused by a finiteQ of each resonator, heats are generated in the resonator because ofthis current, and the resonator is broken by these heats. Here, theresonator “Q” is assumed as:

[0036] Q=2π(accumulated energy)/(energy disappearing in one cycle)

[0037] =2πf(accumulated energy)/(disappearing power).

[0038]FIG. 4 is a circuit diagram of a series-arm SAW resonator.

[0039]FIG. 5 is a concentrated-constant equivalent circuit diagram ofthe series-arm SAW resonator. FIG. 4 is a schematic diagram of theseries-arm resonator S1 410 and FIG. 5 is a concentrated-constantequivalent circuit diagram of the series-arm resonator S 510 of FIG. 4,which is a circuit in which a series circuit of a capacitance Cs1 and areactance Ls1 and a series circuit of a capacitance Cs0 are connected inparallel, and a resistance Rs 520 is connected in series with them.

[0040] In general, from the concentrated-constant circuit of theseries-arm resonator S1 510 of FIG. 5, the resistance part due to afinite Q of the series-arm resonator S1 410 of FIG. 4 is evaluated as aresistance part Rs 520 and calculated. From the concentrated-constantequivalent circuit of the resonator, the resistance part Rs 520 due tothe finite Q of the resonator of FIG. 5 is obtained as follows. Nowassuming that Q of the resonator is a finite Q0, the impedance Z of theseries-arm resonator including Q0 and the admittance Y of theparallel-arm resonator are given by the expression (1).

Z=1/Y=R _(d) +jZ ₀=1/(G _(d) +jY ₀)   (1)

G _(d) ={ωC ₁(1+ω^ 2*L ₁ *C ₁)}/{(1−ω^ 2*L ₁ *C ₁)^ 2}/Q   (2)

Y ₀=ω(C ₀ +C ₁+ω^ 2L ₁ *C ₁ *C ₀)/(1+ω^ 2*L ₁ *C ₁)   (3)

[0041] In case that Q of each resonator is infinite, the impedance ofthe series-arm resonator is jZ0 and the admittance of the parallel-armresonator is jY0. In each resonator, however, because of an actuallyfinite Q, there are a minute resistance part Rd of the series-armresonator and a minute conductance part Gd of the parallel-armresonator.

[0042] From the equivalent LC value of each series-arm resonator of theTx filter 200 of Table 1, assuming that Q of each resonator is 800, theresistance values of the Tx filter 200 at 824 MHz, 836 MHz, and 849 MHzobtained from the expressions (1), (2), and (3) are shown in Table 3.TABLE 3 Frequency (MHz ) 824 836 849 High-frequency Series 1.07 1.935.11 resistance value arm 1,3 Ω Series 2.14 3.77 10.2 arm 2

[0043] When a transmission power is applied to the Tx filter 200, it isdivided into an equivalent power and a reflection power, as illustratedin FIG. 6. This equivalent power is converted into heats by ahigh-frequency resistance of the Tx filter 200 shown in Table 3. Now, inTable 3, paying attention to a frequency 836 MHz, it is found that theresistance value of the series arm 2 is 3.77Ω and it has the resistancethat is substantially double of 1.93Ω of the series arm 1 or 3. That is,it is found that the series arm 2 generates heats that are substantiallydouble of those of the other series arms. When, based on thishigh-frequency resistance of Table 3, the current flowing in each seriesarm is obtained and the applied power to each series arm is obtained,the result is in Table 4. TABLE 4 Frequency (MHz) Series arm 1 Seriesarm 2 Series arm 3 836 (MHz) 0.0596428 0.074404 0.0299007

[0044] From this Table 4, it is found that the highest power is appliedto the series arm 2. That is, it is found that the series arm 2 isweakest to power. This is caused by that the high-frequency resistanceis high as described above.

[0045] In general, the series arms of the Tx filter 200 are set at thesame resonance frequency. But, to reduce the applied power to the seriesarm 2, according to Table 3, if the frequency is changed, since thehigh-frequency resistance of each series arm changes, in the firstembodiment of the present invention, by making the resonance frequenciesof the series arm 1 and the series arm 3 and the series arm 2 of the Txfilter 200 of FIG. 9 differ from each other, the high-frequencyresistance of the series arm 2 of the Tx filter 200 is set small, and byreducing the applied power, it can become a SAW branching filter inwhich the withstand power characteristic has been improved. TABLE 5Resistance and applied power of each series arm by change in resonancefrequency of series arm 2 of first embodiment Series arm 2 Inputimpedance of Tx filter; Input power to input High-frequency equivalentresistance value of each arm of terminal of Tx filter Tx filterResistance Current MHz Conditions R1 R2 R3 R4 R5 real imaginary Powerpart (A) 885 50 1.93 15.00 1.93 15.00 1.93 72.80 28.60 0.92 72.80 0.112terminals 881 50 1.93 15.00 2.18 15.00 1.93 73.60 15.00 0.95 73.60 0.114terminals 877 50 1.93 15.00 2.59 15.00 1.93 65.80 −1.24 0.98 65.80 0.122terminals 873 50 1.93 15.00 3.13 15.00 1.93 47.60 −11.20 0.99 47.600.144 terminals 871 50 1.93 15.00 3.45 15.00 1.93 37.30 −11.20 0.9637.30 0.161 terminals 869 50 1.93 15.00 3.77 15.00 1.93 29.80 −9.04 0.9229.80 0.176 terminals Current and applied power of each arm of Tx filterMHz Conditions P1 (Watt) P2 (Watt) Current 3 (A) P3 (Watt) P4 (Watt)Current 5 (A) P5 (Watt) 885 50 0.02439 0.00246 0.09960 0.01915 0.001930.08825 0.01503 terminals 881 50 0.02499 0.00313 0.09935 0.02152 0.001920.08803 0.01495 terminals 877 50 0.02866 0.00483 0.10391 0.02797 0.002100.09206 0.01636 terminals 873 50 0.04014 0.00930 0.00932 0.04456 0.002780.10572 0.02157 terminals 871 50 0.04983 0.01354 0.13063 0.05887 0.003330.11574 0.02585 terminals 869 50 0.05984 0.01876 0.14072 0.07465 0.003860.12468 0.03000 terminals

[0046] The first embodiment of the present invention is, as shown inTable 5, by setting the resonance frequency of the series arm 2 R3 from869 MHz to the higher frequency side, to reduce the high-frequencyresistance of the series arm 2 R3 form 3.77Ω at 869 MHz to 1.93Ω at 885MHz, after all, to reduce the applied power to the series arm 2.

[0047] So, in the first embodiment of the present invention, as themeans for changing the resonance frequency of the series arm 2,specifically, means for changing inter-electrode pitch of interdigitalelectrodes is employable. For example, in case of a grating reflector,assuming that the grating period (disposition intervals of metallic ordielectric strips) is p and the surface wave velocity is v₀, the centralfrequency f₀ of the reflector can be obtained from an expressionf₀=v₀/2p. That is, in case of a fixed surface wave velocity, theresonance frequency of the series arm 2 can be changed by changing thegrating period. Further, although the frequency characteristic ischanged by the change in resonance frequency of the series arm 2, thischange in frequency characteristic can be rectified by the transverselengths and the number of pairs of the electrodes.

[0048] Table 5 substantially shows effects of the present invention. Ingeneral, every series arm is set at 869 MHz. Table 5 shows theresistance and applied power in relation to each series arm when theresonance frequency of the series arm 2 is changed to 869 MHz to 885MHz. It is found that the applied power to the series arm 2 changes from0.074W to 0.019W by changing the resonance frequency of the series arm 2from 869 MHz to 885 MHz.

[0049]FIG. 2 is a circuit diagram of a Tx filter of the presentinvention.

[0050] The circuit construction of the second embodiment of the presentinvention is a polar construction.

[0051] In the aforementioned first embodiment, paying attention to theseries arm 2 the applied power to which is the highest, the resonancefrequency of the series arm 2 is changed and the resistance of theseries arm 2 is reduced to lower the applied power to the series arm 2.

[0052] Contrastingly, this second embodiment is characterized in that,in the circuit of the Tx filter 200 of FIG. 2, the resistances of theparallel arms 1 and 2 are reduced and the current flowing through theseries arm 2 is branched to the parallel arm 1 to lower the appliedpower to the series arm 2. As the means for reducing the resistances ofthe parallel arms, there are means for reducing the current value perunit area by enlarging the transverse length of teeth, means for alsoreducing the current value per unit area by increasing the number ofpairs, and so on.

[0053] Table 6 shows the resistance value of each single parallel armand the resistance value of each of the series and parallel arms when amultistage filter is assembled. The example of NO. 1 of Table 6 is theconstruction as the standard in the first embodiment. TABLE 6 Resistancevalue (Ω) of Resistance values (Ω) of series and parallel parallel armsarm Series Parallel Series Parallel Series No. 1, 2 arm 1 arm 1 arm 2arm 2 arm 3 1 15 1.93 16.0 3.74 16.0 1.93 0.5 2 10 1.93 11.0 3.71 11.01.93 0.5 3 7.62 1.93 8.62 3.66 8.62 1.93 0.5

[0054] This example in which the resistance value of the parallel arms 1and 2 is 15Ω is a case that Q of the parallel-arm resonator is 200.Table 6 shows data in which the resistance value of only the series arm2 reduces from 3.74Ω to 3.71Ω and further to 3.66Ω as the resistancevalue of the parallel arms 1 and 2 changes from 15Ω to 10Ω and furtherto 7.62Ω. From this data, the principle that, as the resistance value ofthe parallel arm is reduced, the resistance value of the series arm 2reduces accordingly is understood.

[0055] Table 7 shows the resistance value of each single parallel armand the power applied to each of the series and parallel arms when amultistage filter is assembled, of the second embodiment. TABLE 7Resistance Applied powers (Watt) to series and value (Ω) of parallelarms Unit parallel arm Series Parallel Series Parallel Series No. 1, 2arm 1 arm 1 arm 2 arm 2 arm 3 1 15 0.060 0.018 0.076 0.0038 0.031 0.5 210 0.059 0.022 0.064 0.0042 0.024 0.5 3 7.62 0.059 0.024 0.056 0.00440.020 0.5

[0056] No. 1 of Table 7 is, as described above, the construction as thestandard in the first embodiment, in which the applied power to theseries arm 2 is the highest. It is found that, by reducing theresistance value of the parallel arm from 15Ω to 10Ω and further to7.62Ω, the applied power to the series arm 2 can be reduced from 0.076Wto 0.064W and further to 0.056W. Further, it is found that the appliedpower to the series arm 3 also exhibits the same tendency and it can bereduced from 0.031W to 0.024W and further to 0.020W. From this data, theprinciple that, as the resistance value of the parallel arm is reduced,the applied powers to the series arms 2 and 3 reduce accordingly isunderstood.

[0057] As described above, by the means for lowering the applied powerto each series arm in the circuit construction of the polar Tx filter200 of the second embodiment, a high-performance characteristic that cannot be obtained by the construction of the first embodiment can beobtained. It is applicable simultaneously with that of the firstembodiment.

[0058] In the second embodiment, using a technique of reducing thehigh-frequency resistance of the parallel arm and reducing the currentflowing through the series arm to lower the applied power to the seriesarm, the withstand power characteristic of the polar Tx filter 200 isimproved.

[0059] By the matters described in the claims, the present invention isable to reduce the current flowing through the series arm and lower theapplied power to the series arm to suppress the deterioration and breakof the series-arm resonator of the surface acoustic wave filter.

What is claimed is:
 1. A surface acoustic wave filter comprising: anantenna terminal; a transmitting filter coupled to the antenna terminal,the transmitting filter including, a first end series-arm SAW resonatorand a second end series-arm SAW resonator both of which have a firstresonance frequency, a middle series-arm SAW resonator coupled betweenthe first and second end series-arm SAW resonators, the middleseries-arm SAW resonator having a second resonance frequency that ishigher than the first resonance frequency, and a plurality ofparallel-arm SAW resonators; a variable branching line coupled to theantenna terminal; and a receiving filter coupled to the variablebranching line.
 2. A surface acoustic wave filter as claimed in claim 1,further comprising a power amplifier coupled to apply the electricalpower to the transmitting filter.
 3. A surface acoustic wave filter asclaimed in claim 1, further comprising a low noise amplifier coupled tothe receiving filter.
 4. A surface acoustic wave filter as claimed inclaim 1, wherein the receiving filter has a plurality of resonators. 5.A surface acoustic wave filter as claimed in claim 4, wherein thereceiving filter is a π-type filter.
 6. A surface acoustic wave filteras claimed in claim 1, wherein each of the resonators has a plurality ofinterdigital electrodes.
 7. A surface acoustic wave filter as claimed inclaim 1, wherein a distance between the electrodes of the first andsecond end series-arm resonators is different from that of the middleseries-arm resonator.
 8. A surface acoustic wave filter comprising: anantenna terminal; a transmitting filter coupled to the antenna terminal,the transmitting filter including, a first end series-arm SAW resonatorhaving a first resistance value, a second end series-arm SAW resonatorhaving the first resistance value, a middle series-arm SAW resonatorcoupled between the first and second end series-arm SAW resonatorshaving the first resistance value, a first parallel-arm SAW resonatorcoupled to the first end series-arm SAW resonator and the middleseries-arm SAW resonator having a second resistance value that is lowerthan the first resistance value, a second parallel-arm SAW resonatorcoupled to the second end series-arm SAW resonator and the middleseries-arm SAW resonator having the second resistance value; a variablebranching line coupled to the antenna terminal; and a receiving filtercoupled to the variable branching line.
 9. A surface acoustic wavefilter as claimed in claim 8, further comprising a power amplifiercoupled to apply the electrical power to the transmitting filter.
 10. Asurface acoustic wave filter as claimed in claim 8, further comprising alow noise amplifier coupled to the receiving filter.
 11. A surfaceacoustic wave filter as claimed in claim 8, wherein the receiving filterhas a plurality of resonators.
 12. A surface acoustic wave filter asclaimed in claim 11, wherein the receiving filter is a π-type filter.13. A surface acoustic wave filter as claimed in claim 8, wherein eachof the resonators has a plurality of interstitial electrodes each ofwhich has a transverse length.
 14. A surface acoustic wave filter asclaimed in claim 8, wherein each of the transverse length of theelectrodes of the series-arm resonators is different from that of theparallel-arm resonators.
 15. A surface acoustic wave filter as claimedin claim 8, wherein a number of pairs of the electrodes of each of theseries-arm resonators is different from that of the parallel-armresonators.
 16. A surface acoustic wave filter comprising: an antennaterminal; a transmitting filter coupled to the antenna terminal, thetransmitting filter including, a first end series-arm SAW resonator anda second end series-arm SAW resonator both of which have a firstresistance value, a middle series-arm SAW resonator coupled between thefirst and second end series-arm SAW resonators, the middle series-armSAW resonator having a second resistance value that is lower than thefirst resistance value, and a plurality of parallel-arm SAW resonators;a variable branching line coupled to the antenna terminal; and areceiving filter coupled to the variable branching line.
 17. A surfaceacoustic wave filter as claimed in claim 16, wherein the receivingfilter has a plurality of resonators.
 18. A surface acoustic wave filteras claimed in claim 17, wherein the receiving filter is a π-type filter.19. A surface acoustic wave filter as claimed in claim 16, wherein eachof the resonators has a plurality of interdigital electrodes.
 20. Asurface acoustic wave filter as claimed in claim 19, wherein a distancebetween the electrodes of the first and second end series-arm resonatorsis different from that of the middle series-arm resonator.